Switched Modulation of a Radio-Frequency Amplifier

ABSTRACT

Switch-modulation of a radio-frequency power amplifier by-representing the input signal by the I-signal ( 1 ) and Q-signal ( 9 ) of the complex components (I+j−Q), and pulse width modulating the I-signal and the Q-signal separately to create a modulated I-signal pulse sequence ( 3   a ) and a modulated Q-signal pulse sequence ( 3   b ). Further, the pulses corresponding to negative sample values are time-shifted relative the pulses corresponding to positive sample values, and each pulse of the I-signal pulse sequence is delayed by introducing a delaying time shift.

TECHNICAL FIELD

The present invention relates to a method and an arrangement ofswitch-modulating a radio-frequency power amplifier.

BACKGROUND

A power amplifier for a telecommunication system, e.g. for an RF (radiofrequency)-signal transmitter in a base station or a satellite system,may have to amplify frequencies spread over a bandwidth of up to 100MHz, due to simultaneous amplification of multiple carrier signals.Further, it is desirable that an RF amplifier has a high efficiency anddynamic range, as well as a high linearity in order to reduce thedistortion.

A class D power amplifier for example, is a power amplifier in which allpower devices operate in a switched (ON/OFF)-mode, and it may beswitch-modulated e.g. by PWM (Pulse Width Modulation), whereby a highpower efficiency can be obtained. Switched-modulation techniquesnormally involve a conversion of an input signal to a pulse sequencehaving a much higher frequency content than the input signal, asubsequent amplification of the pulse sequence, followed by filtering toremove unwanted spectral components, such as e.g. the carrier frequencyharmonics or modulation noise. The resulting filtered signal correspondsto an amplified replica of the input signal. An advantage of a Class Dpower amplifier is the high power efficiency, which is achieved by thepulse sequence having a fixed amplitude and the switching elements beingeither ON or OFF, which causes a low power dissipation.

In the above-mentioned PWM, the input signal to the amplifier ismodulated to give the pulses in the pulse sequence a duration, i.e. apulse width, that is proportional to the signal amplitude. At the outputfrom the amplifier, the pulse train is filtered with a band-pass filterto obtain its prior shape, without the presence of higher orderharmonics. PWM is commonly used in power electronics, e.g. forcontrolling an electric engine or for power conversion, and further inaudio systems, in which the introduction of PWM reduces the need forcooling of the amplifiers, as well as the size. However, inradio-frequency applications, the use of switch-modulated Class D poweramplifiers is still limited due to the high switching frequencies thatare needed, e.g. in the GHz-range.

Another example is a Class E amplifier, which is a high-efficientswitching power amplifier that is suitable for radio-frequency signals,and consists of a single transistor driven as a switch and a passiveload network.

FIG. 1 is a block diagram illustrating a conventional architecture of aswitch-modulator 2, consisting of a pulse width modulator (PWM) arrangedto modulate a base band input signal 1 and to present a pulse-sequence 3forming a binary-level signal to a power amplifier 4. Since the poweramplifier can be constantly driven at its maximum-efficiency operatingpoint, the overall efficiency for the amplification of a typicalamplitude-varying and phase-varying signal will be very high.Thereafter, the amplified pulse sequence 5 is filtered by a properlydesigned filter 6 tuned around the carrier frequency, preferably by aband pass filter for PWM, in contrast to the low pass filters for PWMcommonly used for power converters or audio system, in order to filterout a correct amplified output signal 7.

In conventional PWM (Pulse Width Modulation), the amplitude of a signalis mapped onto the width of a pulse at each sample, and a single pulseis transmitted by the modulator for each incoming sample. However, e.g.in radio communication systems is required that the phase information ofthe signal is mapped onto some aspect of the pulse, e.g. the position,to thereby create a pulse sequence representing both the amplitude andthe phase of the signal, constituting a PPM (Pulse Position Modulation)that is used together with the PWM, i.e. PWM/PPM. FIG. 2 illustrates howthe amplitude of a signal 1 sampled at T_(s0) is mapped onto the width,i.e. duration, of a modulated pulse 3, and the phase of the signalsample is mapped onto the position of the pulse 3 within the timeinterval between two samples Ts=T_(s1)−T_(s0), i.e. the samplinginterval or sampling period

The technique explained in FIG. 2 is applied in the conventionalarrangement illustrated in FIG. 3, in which the amplitude-part 1 and thephase-part 9 of a baseband signal are modulated by a conventional,combined PWM/PPM 8, by which the amplitude modulates the width of apulse and the phase modulates the position of said pulse within thesampling period of the baseband signal. FIG. 3 further shows a pulsesequence 3 created by the combined PWM/PPM 8 and onto which both theamplitude and phase of the base band signal 1 is mapped, as describedabove. Thereafter, the pulse sequence 3 is amplified by the poweramplifier 4, and the amplified pulse sequence 5 is filtered by the bandpass filter 6, resulting in an amplified base band signal 7 on theoutput.

Related art within the technical field is disclosed e.g. inUS2004/0246060, which describes a modulator for generating a two-levelsignal suitable for amplification by a switching mode power amplifier,such as a Class D or a Class E amplifier.

It is further known within this technical field to combine theabove-described PWM (pulse-width modulation) and PPM (pulse-positionmodulation) with Delta-Sigma (DS) modulation, by presenting theamplitude-part of a complex baseband signal to a 3-levelDelta-Sigma-modulator, while the phase-part of the signal is presentedto a DS-modulator having 8 levels. In DSM (Delta-Sigma modulation), theinput signal to an amplifier is converted to a pulse sequence having afixed pulse width and a frequency that is higher than the carrierfrequency, and the average level of the bit-stream represents the inputsignal level. Normally, the sampling frequency f_(s)=4·carrierfrequency, such as in a f_(s)/4 DS-modulator, resulting in a samplingfrequency of e.g. 8.56 GHz in a frequency band of the 3GPP (The 3^(rd)Generation Partnership Project). In the architecture illustrated in theblock diagram in FIG. 4, the output signals from two Delta-Sigmamodulators 10 a, 10 b are presented to a pulse-width modulating part anda pulse-position modulating part, respectively, of a combined PPW/PPM 8,to form a pulse train that has the combined characteristics ofDS-modulation and PWM/PPM. The DS-modulator produces a noise-shapedspectrum, while the PWM/PPM produces a signal that consists of onlythose pulses that are needed to feed a switched amplifier. A signalproduced by the PWM/PPM does not change as rapidly as a signal comingfrom a DS-modulator, which contains broadband noise with a spectraldensity comparable to that of the useful signal.

In the above-described FIG. 4, the amplitude-part 1 and the phase-part 9of an input base band signal are modulated by two Delta-Sigma modulators10 a, 10 b and thereafter modulated by a combined PWM/PPM 8. Since themapping of the input amplitude onto a pulse width is a non-linearfunction, i.e. a sine-function, an inverse (i.e. arcsine) pre-distorteris needed to obtain a linear output, and this correction ispre-calculated in the correcting calculator 11 in the illustratedarrangement in FIG. 4. The phase information of the input signal isconverted to a pulse position of the pulse sequence 3, e.g. by phasemodulating an oscillator working on the intended carrier frequency, orby up-converting the base band signal to RF and extracting the phaseinformation. Thereafter, the Delta Sigma-modulated and PW/PP-modulatedsignal 3 is amplified by the power amplifier 4 and filtered by theband-pass filter 6.

However, the above-described conventional arrangements, as well asrelated art within the technical field, involve several drawbacks. Forexample, a combined pulse width- and pulse position-modulation with afixed sample period, T, may lead to so-called “wrap-around” of a pulse,which is illustrated in FIG. 5. The pulse representing the sample atT_(s0) is “wrapped” within the sample period, since this first pulsecannot extend over to the next sample interval. Instead, a second pulsewill be transmitted during the next interval, and this second pulse willrepresent the amplitude and phase of the second sample, at T_(s1).

Another drawback with the combined PW/PP-modulation is the timegranularity of a digitally defined pulse width and position, whichrestricts the achievable dynamic range due to quantization noise. Atleast 512 levels would be required for the width or positioning in orderto reach 60-70 dB dynamic range, and this requires a clock frequency anda speed of the digital circuitry that is not achievable today. Thisproblem can be alleviated by means of PW/PP modulation with coarse timegranularity (3/8 levels) in combination with DS-modulation, but thedynamic range of the output signal is still very low, typically in theorder of 40 dB for a single-carrier WCDMA (Wideband Code DivisionMultiple Access), and an extensive filtering is needed to reduce out ofband noise to acceptable levels.

Thus, it still presents a problem to achieve a high-efficientswitch-modulated power amplifier for radio-frequency signals that iscapable of linear amplification of radio-frequency signals over a largebandwidth with a high dynamic range and without “wrap-around” problems.

SUMMARY

The object of the present invention is to address the problem outlinedabove, and to provide an improved switch-modulation of a radio-frequencypower amplifier, involving a low power dissipation, a high linearityover a large bandwidth, as well as a high dynamic range. This object andothers are achieved by the method and arrangement of switch-modulating aradio-frequency power amplifier according to the appended claims.

According to one aspect, the invention provides a method ofswitch-modulating a radio-frequency power amplifier, in which method theinput signal is represented by the I-signal and Q-signal of the complexcomponents (I+j·Q). Further, the method performs sampling and pulsewidth modulation of the I-signal and the Q-signal separately to create amodulated I-signal pulse sequence and a modulated Q-signal pulsesequence, time shifting of the pulses corresponding to negative samplevalues relative the pulses corresponding to positive sample values, anda delay of each pulse of the I-signal pulse sequence by introducing adelaying time shift.

Thereby, a highly efficient and linear amplification of can be obtainedover a large bandwidth and with a high dynamic range. Since themodulation is performed only by pulse width modulation, and not byposition modulation, the phase wrap-around problem is avoided. Further,the delaying time shift of the I-signal pulse sequence will simplify thecombination of the two orthogonal components into the complexrepresentation.

Additionally, new samples values may be interpolated to be mapped on thetime shifted pulses, thereby achieving a more correct mapping of theamplitudes.

The method may also perform a power amplification of the I-signal pulsesequence and the Q-signal pulse sequence separately, a subsequentcombination of the amplified I-signal pulse sequence and the amplifiedQ-signal pulse sequence, and a filtering of the amplified and combinedpulse sequences to create a correct output signal.

The pulse width modulation comprises mapping of the sample amplitude onthe width of a pulse of a pulse sequence, and the amplitude may becorrected to obtain a linear function. The delaying time shift in theI-branch may be 0.25 T_(s), which corresponds to a 90-degree phaseshift, and the time shift of the negative sample pulses relative thepositive pulses may place the negative sample pulses in the second halfof the sampling interval T_(s) and the positive sampling pulses in thefirst half of the sampling interval, e.g. the positive sample pulsesbeing placed at 0.25 T_(s) and the negative sample pulses placed at 0.75T_(s).

Additionally, the filtering may comprise band-pass filtering, in orderto obtain the frequency of interest, and said interpolation may involvelinear interpolation between two adjacent samples, or cubic splineinterpolation.

According to another aspect, the invention provides an arrangement ofswitch-modulating a radio-frequency power amplifier by a separatemodulation of the I-signal and the Q-signal of the complex components(I+j·Q) of an input signal. The arrangement comprises pulse widthmodulators for sampling and modulating the I-signal and the Q-signalseparately to create a modulated I-signal pulse sequence and a modulatedQ-signal pulse sequence, a time shifter for displacing the pulsescorresponding to negative sample values relative the pulsescorresponding to positive sample values within the sampling interval,and a delay unit for introducing a delaying time shift in the I-signalpulse sequence.

The arrangement may further comprise an interpolator for interpolatingnew sample amplitudes to be mapped on the width of each of the timeshifted pulses, and power amplifiers for amplification of the I-signalpulse sequence and the Q-signal pulse sequence separately, a combiner ofthe amplified I-signal pulse sequence and the amplified Q-signal pulsesequence, and a filter for band-pass filtering the combined pulsesequences to create a correct amplified output signal of the frequencyof interest.

A pulse width modulator is arranged to map a sample amplitude on thewidth of a pulse, and may further comprise a correcting calculator forobtaining a linear relationship between the pulse width and the sampleamplitude.

The delaying time shift may be 0.25 T_(s), which corresponds to a90-degree phase shift of the modulated I-signal pulse sequence, theinterpolators may comprises a linear interpolator or a cubic splineinterpolator, and the power amplifiers may be a Class D or a Class Eamplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail and withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram schematically illustrating a conventionalarrangement of a pulse width modulator;

FIG. 2 illustrates mapping of the amplitude and phase of a signal onto apulse;

FIG. 3 is a block diagram illustrating a conventional arrangement of acombined pulse-width modulator and pulse position modulator;

FIG. 4 is a block diagram illustrating a conventional arrangement of acombined Delta-Sigma modulator, pulse-width modulator and pulse positionmodulator;

FIG. 5 illustrates “wrap around” that may occur when the phase of asignal is mapped onto the position of a pulse within a fixed samplingperiod;

FIG. 6 is a block diagram schematically illustrating switch modulationof a power amplifier according to a first embodiment of this invention;

FIG. 7 illustrates mapping of the sample amplitudes of the I-branch- andQ-branch signals, according to this invention;

FIG. 8 illustrates mapping of positive or negative samples of theI-branch- and the Q-branch signals, according to this invention;

FIG. 9 illustrates linear interpolation of a new, time shifted samplevalue, according to an embodiment of this invention;

FIG. 10 illustrates interpolation of a time shifted sample values in theI-branch due to an introduced delay, according to a second embodiment ofthis invention;

FIG. 11 is a block diagram schematically illustrating an arrangement ofswitch-modulating a power amplifier, according to a second embodiment ofthis invention;

FIG. 12 illustrates a band-pass filtered amplified RF-signal, accordingto this invention, and

FIG. 13 is a flow chart of the method of switch-modulating a poweramplifier, according to a first embodiment of this invention.

DETAILED DESCRIPTION

In the following description, specific details are set forth, such as aparticular architecture and sequences of steps in order to provide athorough understanding of the present invention. However, it is apparentto a person skilled in the art that the present invention may bepractised in other embodiments that may depart from these specificdetails.

Moreover, it is apparent that the described functions may be implementedusing software functioning in conjunction with a programmedmicroprocessor or a general purpose computer, and/or using anapplication-specific integrated circuit. Where the invention isdescribed in the form of a method, the invention may also be embodied ina computer program product, as well as in a system comprising a computerprocessor and a memory, wherein the memory is encoded with one or moreprograms that may perform the described functions.

This invention relates to an improved switch-modulation of aradio-frequency power amplifier, and the modulation method is capable ofmapping information regarding both the amplitude and the phase of thebase band signal. Instead of mapping the phase on the position of thepulse between two samples, as in conventional pulse position/pulse phasemodulation, the switch-modulation according to this invention isperformed by two functionally separate pulse-width modulators. The inputbaseband signal is represented by the Cartesian coordinates of a complexnumber, expressed as I+j·Q, of which the real component I represents theIn-phase component and the imaginary component Q represents theQuadrature-phase component of the signal. One of the two separate pulsewidth modulators modulates the in-phase component, hereinafter denotedthe I-signal, and the other modulates the quadrature-phase component,hereinafter denoted the Q-signal. The pulse width modulators map theamplitude of a sample onto the width of a pulse, and negative andpositive sample amplitudes are indicated by a time-shift of the negativesampling pulse relative the positive sampling pulses within the samplinginterval, and, according to a further embodiment of the invention, newinterpolated sample amplitudes are calculated in order to compensate forthe time-shifts.

In order to forward the complex number I+j·Q representing the base bandsignal, and not the real part (I) and the imaginary part (Q) separately,the two signals are preferable transformed to one signal correspondingto I+j·Q after the amplification of the separate signals. Since amultiplication by “j” corresponds to a delay of 90°, the transformationof the two separate signals I and Q involves a delay, i.e. a time-shift,of the I-branch by 0.25 T_(s), of which the sampling period T_(s) is thesampling period at radio frequency (RF), and according to a furtherembodiment of the invention, new interpolated sample amplitudes arecalculated in order to compensate for the time-shift. The delay may beperformed directly in time domain before the two amplifiers, or by meansof physically delaying the signals with a quarter-wave transmissionline, e.g. by a hybrid combiner having an integrated delayingtransmission line section. Thereafter, the output signals from the twopower amplifiers can be combined, e.g. in an ordinary RF branch linecoupler, in order to achieve I+j·Q, which is filtered in order to createa correct, amplified radio-frequency signal at the output.

FIG. 6 is a block diagram illustrating a first embodiment of thisinvention, performing pulse-width modulation of the I-signal branch 1and of the Q-signal branch 9 of a base band signal by means of twofunctionally separate pulse-width modulators 2 a, 2 b. The resulting twoseparate pulse sequences 3 a, 3 b are amplified by two functionallyseparate power amplifiers 4 a, 4 b, and the amplified signals arecombined by a suitable combiner 13, thereby creating a combinedamplified pulse sequence 5, which is filtered by a suitable band-passfilter 6, creating a correct amplified band-pass signal 7.

The negative samples of both the in-phase signal, i.e. the I-signal, andthe quadrature-phase signal, i.e. the Q-signal, are handled byintroducing a suitable time shift of the pulses corresponding to thenegative samples relative the pulses corresponding to the positivesamples, enabling a preserved binary coding. According to a furtherembodiment of the invention, the negative sample pulses are placed inthe second half of the sampling interval T_(s)=T_(s1)−T_(s0), and thepositive samples in the first half. For example, the positive samplesmay be placed at 0.25 T_(s) and the negative samples at 0.75 T_(s)within the sampling interval T_(s).

Further, due to this time shift, the real amplitude of the samples willdeviate from the amplitude mapped onto the pulses, and therefore a newinterpolated sample amplitude is calculated, according to a secondembodiment of this invention.

Thus, according to this invention, two functionally separate poweramplifiers are used to amplify the input signal consisting of onein-phase signal, I-signal, and one quadrature-phase signal, Q-signal.However, in order to obtain a correct complex amplified output radiofrequency signal, the combining of the In-phase (I) and theQuadrature-phase (Q) signals is preceded by a delay of the I-signal,e.g. by a quarter of a sample interval (i.e. by 0.25 T_(s)) after thepulse width modulation. This means that the pulses of the I-signal pulsesequence are time-shifted by 0.25 T_(s) within the sampling interval,such that the positive sample pulses may be placed at 0.5 T_(s) and thenegative sample pulses at 1 T_(s), while the un-delayed Q-signal samplespulses may be placed at 0.25 T_(s) and 0.75 T_(s), respectively. Thisdelay of the I-signal corresponds to a 90-degree phase shift, whichsimplifies a combination of the two orthogonal components into thecomplex representation I+j·Q.

Thus, the solution according to this invention is to apply PWM (pulsewidth modulation) on the Cartesian complex components and Q representingthe input baseband signal, as illustrated in the block diagram of FIG.6. A positive real signal, having only a positive component, can bemapped (as shown in FIG. 2) by the amplitude onto the width of the pulsee.g. by placing the pulse between two adjacent samples. However, acomplex signal comprising both a real I-signal-component and animaginary Q-signal-component is represented by both an I-signal and aQ-signal, according to this invention. FIG. 1 illustrates theswitch-modulation according to an exemplary embodiment of thisinvention, involving a separate pulse width modulation of the I-signaland the Q-signal by representing the positive amplitude of the samplesat T_(s0) by the width of the pulses, and by placing the pulses centeredaround 0.5 T_(s).

FIG. 8 illustrates the handling of negative samples, according to thisinvention, by time shifting of the negative sample pulses relative thepositive sample pulses. A negative sample amplitude is mapped on a pulseby placing the pulse e.g. in the second half of the sampling period,which is illustrated in FIG. 8 by the sample in the I-branch at T_(s1)being placed on 0.75 T_(s), and by placing a pulse associated with apositive amplitude in the first half of the sampling period, as theillustrated sample at T_(s0) in the I-branch, which is placed or 0.25T_(s). Correspondingly, a pulse associated with a positive amplitude inthe Q-branch, e.g. as the illustrated sample at T_(so) and at T_(s1), isplaced in the first half of the associated sampling period, such as e.g.on 0.25 T_(s). Thus, a complex signal is transformed into a real signalby assigning the pulses to slightly different positions within thesample interval T_(s).

Obviously, the above described positions of 0.25 T_(s) for positivesample values and 0.75 T_(s) for negative sample values are onlyexamples of suitable positions within the sample interval, and manyother positions are possible. Further, the positive sample values mayalternatively be placed in the second half of the sampling period, andthe negative in the first half, and on other suitable positions than on0.25 T_(s) and on 0.75 T_(s), such as e.g. on 0.30 T_(s) and on 0.80T_(s), or on any other suitable position having a time difference of 0.5T_(s) between positive and negative samples.

FIG. 9 illustrates handling of the offset of the negative pulsesrelative the sampling events, according to a second embodiment of thisinvention, due to the introduced time shift. If a pulse representing anegative sample at t=T_(so) is time shifted from its original positionat 0.25 T_(s), by half a sample period, i.e. by 0.5 T_(s), to a newposition at 0.75 T_(s), the pulse width of the pulse does not representthe amplitude corresponding to its new position. Therefore,interpolation is performed to obtain an interpolated time shifted samplevalue Y_(int) at t=0.5 T_(s) to be mapped on the pulse width, asillustrated in FIG. 9. The interpolation may be performed by anysuitable interpolation method, e.g. by a simple linear interpolationbetween the two nearest samples, or by cubic spline interpolation.According to an exemplary embodiment of this invention, which isillustrated in FIG. 9, interpolation of a time shifted negative samplevalue is performed by interpolating a time shifted sample at 0.5 T_(s),and the interpolation method is linear interpolation using the formula:

$y_{neg} = {\frac{1}{2} \cdot \left( {y_{n + 1} + y_{n}} \right)}$

In order to forward the complex number I+j·Q representing the base bandsignal, the two separate pulse sequences representing the modulated I-and Q-signals are transformed, or combined, into one signalcorresponding to I+j·Q after the amplification of the separate signals.This combining is preceded by the delay of the I-branch by 0.25 T_(s),since a multiplication by “j” corresponds to a phase shift of 90°,corresponding to a quarter of a period. Thereafter, the output signalsfrom the two amplifiers can be combined e.g. in an ordinary RF branchline coupler. However, due to this delay, a similar interpolation as theone for the negative pulses is preferably performed for the new delayedpulse positions, both positive and negative, of the I-branch, accordingto a second embodiment of this invention, as illustrated in FIG. 10.

In the I-branch of FIG. 10, a pulse corresponding to a n positiveun-delayed T_(so)-sample would be placed on 0.25 T_(s). By introducingthe delay by 0.25 T_(s) in the I-branch in order to simplify thecombination of the I-branch and the Q-branch, the pulse corresponding toa delayed T_(so)-sample is time shifted and placed on 0.5%. However, inorder to obtain a more correct mapping of the amplitude on the pulsewidth of the pulse on 0.5 T_(s), the sample value at 0.25 T_(s) isinterpolated by any suitable interpolation method, e.g. by linearinterpolation between the two nearest samples, at T_(s0) and T_(s1),thereby obtaining T_(soint).

The consecutive illustrated sample at T_(s1) in the I-branch isnegative, and a corresponding un-delayed pulse would e.g. be placed on0.75 T_(s). However, due to the introduced delay by 0.25 T_(s), thecorresponding pulse will be time shifted and placed on 1 T_(s). In orderto obtain a more correct amplitude mapping on the pulse width at 1T_(s), the sample value of the I-branch at 0.75 T_(s) is interpolated byany suitable method, e.g. by a simple linear interpolation between thenearest two samples, T_(s1) and T_(s2), thereby obtaining T_(s1int).

Thus, according to an exemplary embodiment of this invention, if a firstsample is indicated as sample n, and consecutive sample is indicated bysample n+1, the interpolated time-shifted new first sample values of Qand I are:

Q_(pos)=Q_(n), the time shift=0 for a positive sample value in theun-delayed Q-branch.Q_(neg)=Q_(n)+0.5(Q_(n+1)−Q_(n)), the time shift is 0.5 T_(s) for anegative value in the Q-branch.I_(pos)=I_(n)+0.25(I_(n+1)−I_(n)), the time shift is 0.25 T_(s) for apositive sample value in the I-branch with an introduced delay.I_(neq)=I_(n)+0.75(I_(n+1)−I_(n)), the time shift is 0.75 T_(s) for anegative value in the I-branch with an introduced delay.However, the interpolation must be re-calculated accordingly if anyother suitable time shift is used.

FIG. 11 is a block diagram illustrating a second embodiment of theinvention, comprising two functionally separate pulse width modulators 2a, 2 b, for modulating the separate I-signal branch 1 and the separateQ-signal branch 9 of the complex representation I+j·Q of an input baseband signal. In order to obtain linearity, a correction, e.g. by anarcsine-function, may be performed by a correction calculator (notillustrated in the figure) or directly in the two modulators 2 a, 2 b.The separate pulse width modulators create two separate pulse sequencescorresponding to a modulated I-signal and Q-signal, respectively, and apulse in the pulse sequence corresponding to negative sample value istime shifted by the time shifters 14 a, 14 b within the samplinginterval relative a pulse corresponding to a positive sampling value.Further, in the pulse sequence 3 a representing the modulated I-signal,a delay is introduced by a delay unit 12, preferably by 0.25 T_(s),which corresponds to a multiplication by j, in order to simplify thecombination of the I-signal branch and Q signal branch to re-create thecomplex base band signal I+j·Q. In order to improve theswitch-modulation, an interpolation of new time shifted and delayedsample values is performed in interpolators, 15 a, 15 b, prior to themapping of the amplitudes on the pulse width in the two modulators 2 a,2 b. The first interpolator 15 a for the I-branch interpolates newsamples values due to the time shift of the negative sample values andthe introduced delay in the I-branch, while the second interpolator 15 binterpolates new sample values only due to the time shift of thenegative values. The interpolated and time shifted pulse sequences areamplified by the power amplifiers, 4 a, 4 b, and combined in thecombiner 13. Thereafter, the amplified and combined signal is filteredby a suitable band pass filter 6 in order to achieve an amplified output7 of the input base band signal.

FIG. 12 illustrates a magnified view of a filtered-out spectrum of abase band signal that is switched-modulated and amplified, according tothe method and arrangement of this invention. The filtered-outfrequencies represent the amplified original complex base band signal,and the two-carrier nature of the filtered-out signal indicates that agood dynamic range is achieved.

FIG. 13 is a flow chart illustrating a method of switch-modulating aninput signal to a radio frequency amplifier, according to a firstembodiment of this invention. The sampled input base band signal in theCartesian complex coordinates I+j·Q, is represented by one I-signalbranch and one Q-signal branch, in step 141. Thereafter, in step 142,the I- and Q-signals are input to two separate pulse width modulatorsfor mapping of the amplitude of each sample on the width of a pulse in apulse sequence.

A time shift of the pulses representing negative amplitudes relative thepulses representing positive amplitudes is performed in step 143,placing the positive and negative sample pulses on two differentpositions within the sample period, e.g. positive sample pulses on 0.25T_(s) and negative sample pulses on 0.75 T_(s). In order to facilitate asimple combining of the separate I signal and Q signal branches again tore-create the complex representation I+j·Q, the pulse sequencerepresenting the I-signal branch is delayed in step 144, e.g. by 0.25T_(s), which corresponds to a multiplication with j.

In step 145, the pulse sequences are amplified in two separateamplifiers, and in step 146, the amplified pulse sequences are combinedto create a complex representation I+j·Q. In step 147, the amplified andcombined pulse sequence is filtered in a suitable band pass filter, toobtain an amplified base band signal.

According to a second embodiment of the invention, an interpolation ofnew time shifted samples of the I-signal and the Q-signal is performedprior to the mapping of the samples on the width of the pulses, due tothe time shifting illustrated in the steps 143 and 144 in FIG. 13. Theinterpolation is performed in order to calculate new sample values forthe negative samples in the Q-branch that are time shifted e.g. from0.25 T_(s) to 0.75 T_(s), and for the samples in the delayed I-branch,that are time shifted e.g. from 0.25 T_(s) to 0.50 T_(s) (the positivesamples), or from 0.75 T_(s) to 1 T_(s)s (the negative samples).According to a preferred embodiment, the interpolation comprises linearinterpolation, but it may alternatively comprise any other suitableinterpolation method, e.g. cubic spline interpolation.

The presented solution enables the use of pulse width modulation inswitched radio frequency power amplifiers, achieving a large powerefficiency. Further, the pulse width modulation is employed withoutrequiring any up-conversion to a radio frequency, in spite of a veryhigh sampling frequency corresponding to the radio frequency carrierfrequency. Further advantages are that phase wrap-around is avoided,while a larger dynamic range than in a combined Delta-Sigma and pulsewidth modulation can be obtained.

While the invention has been described with reference to specificexemplary embodiments, the description is in general only intended toillustrate the inventive concept and should not be taken as limiting thescope of the invention.

1-21. (canceled)
 22. A method of switch-modulating a radio-frequencypower amplifier, wherein an input signal into the power amplifier isrepresented by an I-signal and Q-signal of complex components (I+j·Q),the method comprising: sampling and pulse width modulating the I-signaland the Q-signal separately to generate a modulated I-signal pulsesequence and a modulated Q-signal pulse sequence; time shifting thepulses corresponding to negative sample values relative to the pulsescorresponding to positive sample values; and delaying each pulse of theI-signal pulse sequence by introducing a delaying time shift.
 23. Themethod of claim 22 further comprising interpolating new samples valuesto be mapped on the time shifted pulses.
 24. The method of claim 22further comprising: power amplifying the I-signal pulse sequence and theQ-signal pulse sequence separately; combining the amplified I-signalpulse sequence and the amplified Q-signal pulse sequence; and filteringthe amplified and combined pulse sequences to generate a correct outputsignal.
 25. The method of claim 22 wherein pulse width modulating theI-signal and the Q-signal separately comprises mapping a sampleamplitude on the width of a pulse of one of the pulse sequences.
 26. Themethod of claim 25 wherein the sample amplitude is corrected to obtain alinear function.
 27. The method of claim 22 wherein the delaying timeshift in the I-branch is 0.25 of a sampling interval T_(s), andcorresponds to a 90-degree phase shift.
 28. The method of claim 22wherein time shifting the negative sample pulses relative to thepositive sample pulses comprises placing the negative sample pulses in asecond half of a sampling interval T_(s), and placing the positivesampling pulses in a first half of the sampling interval T_(s).
 29. Themethod of claim 23 wherein interpolating new sample values comprises alinear interpolation between two adjacent samples.
 30. The method ofclaim 23 wherein interpolating new sample values involves cubic splineinterpolation.
 31. A circuit for switch-modulating a radio-frequencypower amplifier by separately modulating an I-signal and a Q-signal ofcomplex components (I+j·Q) of an input signal, the circuit comprising:one or more pulse width modulators configured to sample and modulate anI-signal and a Q-signal separately to generate a modulated I-signalpulse sequence and a modulated Q-signal pulse sequence; a time shifterconfigured to displace, within a sampling interval, those generatedpulses that correspond to negative sample values relative to thosegenerated pulses that correspond to positive sample values; and a delayunit configured to introduce a delaying time shift in the I-signal pulsesequence.
 32. The circuit of claim 31 further comprising an interpolatorconfigured to interpolate new sample amplitudes to be mapped on a widthof each time shifted pulse.
 33. The circuit of claim 31 furthercomprising: two power amplifiers configured to amplify the I-signalpulse sequence and the Q-signal pulse sequence separately. a combinerconfigured to combine the amplified I-signal pulse sequence with theamplified Q-signal pulse sequence; and a filter configured to band-passfilter the combined pulse sequences to generate a correct amplifiedoutput signal.
 34. The circuit of claim 31 wherein each said pulse widthmodulator is configured to map the sample amplitude onto the width of apulse.
 35. The circuit of claim 34 further comprising a correctingcalculator configured to obtain a linear relationship between a width ofthe pulse and a sample amplitude.
 36. The circuit of claim 31 whereinsaid delaying time shift is 0.25 of a sampling interval T_(s), andcorresponds to a 90-degree phase shift of the modulated I-signal pulsesequence.
 37. The circuit of claim 32 wherein said interpolatorcomprises a cubic spline interpolator.
 38. The circuit of claim 31wherein the power amplifier is one of a Class D amplifier and a Class Eamplifier.
 39. The circuit of claim 32 wherein said interpolatorcomprises a linear interpolator.